1. Field of the Invention
The present invention relates to a marine vehicle, more particularly to a marine vehicle having a pre-swirl generator capable of generating pre-swirl flow to improve propulsive efficiency of the marine vehicle.
2. Description of the Related Art
Referring to FIG. 1, an existing marine vehicle 1 comprises a hull 11, and a propeller 12 that is mounted on the hull 11 and disposed under waterline 10. When the propeller 12 rotates, propeller blades thereof produce a propulsive force that drives the marine vehicle 1 to move forward or rearward.
Due to the working mechanism of the propeller, the propeller 12 produces a large amount of turbulence and dissipation loss while generating the propulsive force, thus reducing the effective propulsive force of the propeller 12. If the rotational speed of the propeller 12 is increased to improve propulsion, more turbulence and dissipation loss will result, thereby failing to enhance propulsion and advanced speed of the marine vehicle 1.
Referring to FIG. 2, in order to improve propulsion of the propeller 12, Taiwanese Patent Publication No. 200503920 discloses a flow guiding shroud 13 provided around the propeller 12 so that dissipation energy is gathered and converted into propulsive energy, thus improving the propulsion of the propeller 12. However, the flow guiding shroud 13 that covers the propeller 12 increases resistance to the rotation of the propeller 12, and the resistance tends to cancel or even exceed the propulsive energy enhanced by the flow guiding shroud so that the required propulsive horsepower may increase rather than decrease. Moreover, the flow guiding shroud 13 is not suitable for a large-sized marine vehicle since the cost thereof is high.
On the other hand, in order to reduce vibration of a marine vehicle, one of the countermeasure is to fit a pair of triangular hydrofoils on a port quarter and a starboard quarter of the marine vehicle to change an aft flow field and to reduce an exciting force of the propeller 12 behind the non-uniform distributed stern wake conditions. However, based on experiments conducted by the Hamburg Ship Model Basin (HSVA), such an arrangement can consume up to 2%-6% of propulsive horsepower for the marine vehicle.